Currently, in the Third Generation Partnership Project (3GPP), in order to accommodate explosively increasing traffic, various technologies for increasing the capacity of cellular systems are being investigated. In the future, it is predicted that a capacity of about 1000 times the current capacity will be necessary. Technologies such as multi-user multi-input multiple-input multiple-output (MU-MIMO) and coordinated multipoint (CoMP) are considered to increase the capacity of cellular systems to only about several times the previous capacity. Therefore, a breakthrough technique is necessary.
For example, as a technique for significantly increasing the capacity of cellular systems, a base station that uses a directional antenna including multiple antenna elements (for example, about 100 antenna elements) and performs beamforming is considered. Such technology is a form of technology called large-scale MIMO or massive MIMO. According to such beamforming, a half width of a beam becomes narrower. That is, a sharp beam is formed. In addition, when the multiple antenna elements are arranged on a plane, it is also possible to form a beam in a desired three-dimensional direction.
Various beamforming technologies are proposed. For example, in Patent Literature 1, technology for implementing beamforming by a base station even when frequency bands of an upstream channel and a downstream channel are different is disclosed.